This invention relates to the art of particle size distribution analyzation and in particular to apparatus and method for making particle size distributions more narrow and free from artifacts.
To the extent that it might be necessary to understand fully the teachings of the invention herein, the following patents are incorporated by reference:
1. U.S. Pat. No. 2,656,508 to Wallace H. Coulter, PA0 2. U.S. Pat. No. 3,668,531 to Walter R. Hogg.
U.S. Pat. No. 3,668,531 to W. R. Hogg for "Pulse Analyzing Apparatus", relating to art similar to that of this invention, uses apparatus which is responsive only to the amplitude formed at the center of a pulse.
The apparatus disclosed in U.S. Pat. No. 2,656,508 operates on a principle discovered by Wallace H. Coulter, known as the Coulter principle. In accordance with this principle, a sample quantity of a fluid suspension of the particles to be studied is caused to pass through a constricted path and the presence or absence of a particle gives rise to a detectable change in the electric characteristics of the path. That path is constructed so as to provide a region in which the electric field is substantially uniform or homogeneous and, at least in the case that the suspending fluid is a conductive liquid, the change is very nearly proportional to the volume of the particle which caused it.
In the usual commercial embodiment of the Coulter principle, the constricted path is achieved by a fine aperture in a wafer of insulating material which separates the sample suspension into two portions, each of which is connected to an electronic detector by means of a sensing electrode. These electrodes also supply the electrical current. The effective aperture bore usually will include fluid in regions slightly bulging out of the ends of the aperture convexly where the current density is relatively high, as will be seen hereinafter.
The passage of particles gives rise to electric signals which, when counted, give an accurate indication of the number of particles and, through the use of some form of amplitude discriminating means, the particles are classified according to their size by classifying the different amplitudes of pulses.
In order to obtain an electric signal, according to the U.S. Pat. No. 2,656,508, an electric current is established in the aperture. This is done by providing the aperture in a wall of an insulating material between two bodies of liquid, at least the upstream body having the sample suspension therein, immersing electrodes in each body of liquid, and connecting a suitable current source to the electrodes. The fluid is caused to flow from one fluid body through the aperture to the other body and, as it flows, the particles are carried through the aperture at a rapid rate. The impedance changes which are produced by the passage of each particle are detected by some form of electronic detector connected to the electrodes. In commercial versions of the "Coulter Counter" the electric signals produced are counted, displayed on a cathode ray oscilloscope, classified by various fixed and/or variable threshold circuits, recorded, and so on.
Generally, in the use of the apparatus for counting and sizing, as in most routine medical and biological work, the linearity of response of signals to particle sizes is not as important as in specific medical and biological research and in industrial studies. In the latter field, the dynamic range of particle size is quite great and often the entire spectrum of sizes must be studied.
Studies were made with a view towards ascertaining why there was a discrepancy in certain particle sizing investigations carried on in which the sizes of the particles as measured in accordance with the Coulter principle seemed greater than they should have been.
It was found that the center of a pulse is its more accurate part, since there could be false peaks at the beginning and end of pulses, caused by certain phenomena. For most purposes, the center of the signal pulse represents the detecting region having the most uniform field. In accordance with that finding, U.S. Pat. No. 3,668,531 suggests a solution to the problem of accurate particle analysis by means of apparatus which provides size information based upon a measurement of the amplitude of only the center of each pulse.